Banks initially sat on the Committee of Military Affairs while serving in Congress. He bucked the Democratic party line by voting against the Kansas, Nebraska Act in 1854. This bill overturned the 1820 Missouri Compromise. He used his parliamentary skills to keep the bill from coming to a vote despite long stated support for Manifest Destiny. In 1854, he formally joined the so-called Know Nothing cause, officially named American Party since 1855. He was considered one of the political leaders of the movement alongside Henry Wilson and Governor Henry J. Gardner. None of the three supported its extreme anti-immigrant positions. In 1855, Banks agreed to chair the convention of a new Republican Party. His platform intended to bring together antislavery interests from Democrats, Whigs, Free Soilers, and Know Nothings. During summer 1855, Banks spoke at an antislavery rally in Portland, Maine. He expressed that the Union did not necessarily need to be preserved under certain conditions. Future opponents would repeatedly use these words against him, accusing him of disunionism. At the opening of the 34th U.S. Congress in December 1855, only 35% of the House were Democrats. Representatives from several parties opposed to slavery's spread gradually united behind Banks. The coalition supporting him formed by his American Party and the Opposition Party marked the first form of coalition in congressional history. This victory raised Banks's national profile significantly. He gave antislavery men important posts in Congress for the first time. He cooperated with investigations of both the Kansas conflict and the caning of Charles Sumner on the floor of the Senate. Former Speaker Howell Cobb called him in all respects the best presiding officer he had ever seen. Banks played a key role in bringing forward John C. Frémont as a moderate Republican presidential nominee in 1856. He refused the Know Nothing nomination which went instead to former President Millard Fillmore.